Calculate difference in percentage between 2 hex colors

colormeter.js

function color_meter(cwith, ccolor) {
    
    if (!cwith && !ccolor) return;
    
    var _cwith  = (cwith.charAt(0)=="#") ? cwith.substring(1,7) : cwith;
    var _ccolor = (ccolor.charAt(0)=="#") ? ccolor.substring(1,7) : ccolor;
    
    var _r = parseInt(_cwith.substring(0,2), 16);
    var _g = parseInt(_cwith.substring(2,4), 16);
    var _b = parseInt(_cwith.substring(4,6), 16);

    var __r = parseInt(_ccolor.substring(0,2), 16);
    var __g = parseInt(_ccolor.substring(2,4), 16);
    var __b = parseInt(_ccolor.substring(4,6), 16);
    
    var p1 = (_r / 255) * 100;
    var p2 = (_g / 255) * 100;
    var p3 = (_b / 255) * 100;

    var perc1 = Math.round((p1 + p2 + p3) / 3);
    
    var p1 = (__r / 255) * 100;
    var p2 = (__g / 255) * 100;
    var p3 = (__b / 255) * 100;
    
    var perc2 = Math.round((p1 + p2 + p3) / 3);
    
    return Math.abs(perc1 - perc2);   
}

Would there be a way to do something similar to this with HSV values? How can you be accurate in your difference calculation when accounting for different lightning values.

Might as well live demo it, huh?
https://plnkr.co/edit/ERaf37?p=preview

Let's say that we found the percent difference between two blues to be 20 percent. Now we have a green color and we want it to be exactly the same percentage of difference (20 percent). Is there a way to rewrite the code/formula to find this?
Sort of like: x - y = 20%,
z +/- 20% = ?

It's been a while since math class, but isn't is redundant to multiply both the numerator and the denominator by 100?

Also I'm curious if this method is documented somewhere as a standard method?

I don't see it listed on the Wikipedia page for Color Difference on the topic The easiest method shown there is the Euclidean colour difference formula: